It became especially useful, and even essential, to wireless radio operators on both military and civilian ships at sea before the development of advanced single-sideband telephony in the 1960s.
Radio communications, prior to the advent of landlines and satellites as communication paths and relays, was always subject to unpredictable fade outs caused by weather conditions, practical limits on available emission power at the transmitter, radio frequency of the transmission, type of emission, type of transmitting antenna, signal waveform characteristics, modulation scheme in use, sensitivity of the receiver and presence, or lack of presence, of atmospheric reflective layers above the earth, such as the E-layer and F-layers, the type of receiving antenna, the time of day, and numerous other factors.
This was particularly true of CW radio circuits shared by a number of operators, with some waiting their turn to transmit.
If the receiver hears the sender in a "loud and clear" condition, the response would be "QRK 5X5": All of which requires less circuit time and less "pounding" on the key by the sending operators.
According to ACP-125(F), paragraphs 103 and 104, in radio communication among Allied military units: Some assert that the use of Q codes and Z codes was not intended for use on voice circuits, where plain language was speedy and easily recognizable, especially when employing the character recognition system in use at the time, such as ALPHA, BRAVO, CHARLIE, etc.